Epoxy resins have a spectrum of properties that are well adapted for use in molding compositions, coatings, adhesives, fiber reinforced laminates, composites, engineering plastics and various speciality applications such as potting resins and mortar. Included in this spectrum of properties are good corrosion and solvent resistance, good adhesion, good electrical properties, good dimensional stability, hardness, low shrinkage on cure and many other beneficial properties. Unfortunately, epoxy resins are brittle.
Several attempts have been made to produce an epoxy resin composition having the desirable epoxy resin properties and reduced brittleness. Early attempts to accomplish this objective involved dissolving a reactive liquid polymer such as a carboxyl-terminated butadiene-acrylonitrile copolymer into an epoxy prepolymer as discussed in the background sections of U.S. Pat. No. 4,707,529 and Japanese Patent (Kokai) No. SHO 58[1983]-17160. Incorporation of the reactive liquid polymer leads to the formation of discrete elastomer particles or domains which toughened the cured epoxy resin. However, the resulting composition will exhibit a lower softening temperature and a lower glass transition temperature than the unmodified epoxy resin. While improved mechanical properties, particularly toughness, are generally obtained, it is difficult to control the extent and consistency of the improvement. This is primarily due to the difficulty of controlling the morphology of this particular composition.
Another attempt to improve the toughness of polyepoxides with liquid polymers involves incorporating liquid acrylate rubbers into epoxy resin compositions. Such compositions are the subject of European Patent Application No. 78,527. However, the acrylate rubbers are generally soluble in the polyepoxide continuous phase at temperatures above about 51.degree. C. and in some cases even at room temperature. As a result, undesirable softening of the cured epoxy resin composition can readily occur. Also, the polymeric phases do not separate until the epoxy cures, and the dissolving and reprecipitation of these phases are sensitive to the manner in which the curing takes place. It is therefore difficult to control the size of the particles of the dispersed modifying phase, and hence, the morphology of the cured composition.
Another attempt to improve polyepoxide compositions involved the incorporation of a normally solid, selectively hydrogenated, modified block copolymer in the composition as taught by Japanese Patent 58-17160. The block copolymer contained at least one vinyl aromatic polymer block (styrene) and at least one hydrogenated conjugated diolefin polymer block (butadiene). The polymer was modified by grafting dicarboxylic acid or carboxylic acid derivative (e.g. anhydride) functionality into butadiene blocks. Functionalization of the styrene-butadiene block copolymers in the butadiene blocks increases the compatibility of the functionalized block copolymer with the epoxy resin, and therefore results in a more stable suspension of block copolymer particles before curing. Applicant has found that these compositions have, after curing, excellent toughness and retain favorable properties of the cured epoxies, but suffer from the disadvantage of being extremely difficult to mold and handle when the amount of rubber incorporated in the composition exceeds about 10% by weight. It is desirable to have an epoxy resin elastomeric block copolymer composition with lower viscosity at the high shear rates found in epoxy resin processing equipment.
A styrene-butadiene block copolymer toughened epoxy resin composition with block copolymer particle sizes within the range of 0.05 to 20 microns is taught in Japanese Patent Application 63-308,027. Small particle sizes are obtained by dissolving the block copolymer and epoxy in a co-solvent and then evaporating the co-solvent. Functionalization of the block copolymer is taught to be optional. The examples of the invention all include block copolymers functionalized by grafting an alpha-beta unsaturated carboxylic acid anhydride to selectively hydrogenated block copolymers. Functionalization of vinyl aromatic-conjugated diolefin block copolymers by this process is known to incorporate the functionality predominantly in the conjugated diolefin blocks. Applicant has found that solution blending styrene-butadiene block copolymers which have been functionalized by this process results in small particle sizes, but the particles have a wide range of sizes and the rate at which the solvent is removed has a large effect on the size and shape of particles in the epoxy resin composition. Further, at block copolymer loadings above 10%, the epoxy resin compositions containing butadiene block functionality generally have a high shear viscosity which is too high to permit processing by normal epoxy resin processing means.
U.S. Pat. No. 4,778,851 describes epoxy compositions having dispersed shell-core elastomers produced by emulsion polymerization of a crosslinked conjugated diene to which is grafted a shell of a functionalized polymer. The "shells" are said to stabilize the rubbery particles in the epoxy resin composition, and upon curing of the epoxy to react with the epoxy to co-cure the rubbery particles into the resin phase. The grafting of the shell onto the core is a time-consuming polymerization which requires careful control, and the uniformity of the resultant rubber particles within the cured epoxy resin is dependent upon providing a latex of crosslinked rubbery particles of uniform size. Additionally, a stabilizer such as sodium dodecylbenzene sulfonate soap is required to stabilize the rubber particles prior to grafting the "shell". These stabilizers can be undesirable impurities in the finished epoxy resin composition. A process without these drawbacks is desirable.